1. Field of the Invention
The present invention relates to image processing, especially to the usage of a frame memory and a cache memory for storing pixel data pieces of an image.
2. Description of the Related Art
Conventional apparatuses for performing image processing have adopted a method of storing image's pixel data pieces generated by a graphics engine once in the cache memory having a limited storage capacity, and subsequently storing, in the frame memory, the pixel data pieces stored in the cache memory.
While a frame memory is required to have a large storage capacity, a high-speed memory with a large storage capacity is very expensive. Accordingly, drawing apparatuses use a low-speed and inexpensive memory, such as a DRAM (Dynamic Random Access Memory), for the frame memory in the light of cost performance. Since the drawing process requires a longer time period if a low-speed memory is used as a frame memory, a high-speed memory is provided, as a cache memory, between the graphics engine and the frame memory, which thereby reduces the time for writing pixel data pieces to the frame memory. Because a high-speed memory is expensive as stated above, limiting the storage capacity of the cache memory reduces an increase of costs.
Images can be stored in the frame memory without using a cache memory. In this case, however, the graphics engine devotes much of the processing power to writing pixel data pieces to the frame memory, which makes it difficult to perform other tasks such as creating the next drawing data. The overall storage speed therefore slows down in the case of storing multiple sets of drawing data. Providing a cache memory between the graphics engine and the frame memory achieves high-speed writing from the graphics engine to the cache memory. This allows the graphics engine, for example, to prepare the next drawing data during the time until data once stored in the cache memory is written to the frame memory, leading to speed up in the overall writing process.
In the case of using a cache memory, a conventional technique improves its writing efficiency by reducing the number of writing operations from the graphics engine to the cache memory based on a shape of the target to be drawn.
In this regard, Japanese Laid-Open Patent Application Publication No. H1-262585 discloses a technological method of writing data to the cache memory for improving the efficiency. The method reduces the number of writing operations to the cache memory by expanding the two dimensional array of the cache memory, based on the shape of the image to be drawn, in the x-axis direction in the case that the drawing target is horizontally long, or in the y-axis direction in the case that the drawing target is vertically long.
According to the disclosed method, the time period required for the process of writing data from the graphics engine ultimately to the frame memory is shortened by reducing the number of writing operations from the graphics engine to the cache memory to be less than conventional, as described above. However, there is a call for a further shortening of the writing period of drawing apparatuses since an increase in the transfer time resulting from the enhancement of pixel quality is expected.